I think markandrew has the best point. Thanos is trying to impress somebody (mind you, Death likely doesn't care too much about DDR). Darkseid probably wouldn't be interested in DDR unless someone told him it held the secret to Anti-life. Even then, he'd probably get one of his lackeys to check it out for him. I suspect Glorious Godfrey is probably pretty good at the DDR.

Thanos has that self-defeating personality problem. That'll kill you at DDR. He'll start skipping steps that he doesn't think he can hit, confident that he can make up the difference, and in the end it just won't happen. It'll be like "Casey at the Bat" (if Casey loved dance music and worshipped death).

If Thanos ain't got no soul, then I've got to give it to Darkseid. He could probably use his Omega Vision to hit each arrow perfectly, or something. That Omega vision seems to fit just about any task, after all.